Tubuloglomerular feedback (macula densa, sensor for volume and salt flow in the descending convoluted tubule, activates the juxtoglomerular cells of the afferent arteriole to secrete renin into the blood stream. Renin converts angiotensinogen to angiotensin I which is converted to angiotensin II which is a vasoconstrictor. This increases MAP systemicaly which increases pressure in the kidneys.)

The kidney's primary functions are to maintain fluid volumes of the body by regulating salt balance and to maintain the osmolarity of the body by regulating water balance.

GFR is regulated independent of MAP ranging from 80-180 mmHg because of autoregulation.

Autoregulation can include myogenic responses, tubular-glomerular feedback and reflex feedback.

Renal blood flow is regulated independent of MAP between 80-180 mmHg by changing the resistance of the renal arteries and arterioles.

Filtration fraction is the ratio of GFR to renal plasma flow (RPF). In a normal human kidney this is usually 0.2.

Reabsorption moves filtered solutes from the renal tubule to the blood.

Reabsorption of solutes occurs predominantly within the PCT. Some reabsorption of Na+, K+ and Cl- also occurs within the TAL. Only 1% of Na+ reabsorption occurs in the DCT and CD and is regulated by aldosterone.

Most solutes cross the epithelium by secondary active transport with Na+. Urea and Cl- reabsorption is by facilitated diffusion in the distal 1/3 region of the PCT.

Reabsorption within the PCT occurs iso-osmotically with water following Na+.

Secretion of organic compounds occurs in the PCT by secondary active transport.

Secretion of K+ occurs in the DCT and CD in response to increased Na+ delivery or high fluid flow.

Clearance (C) is the excretion rate of a substance. C equals GFR when the substance is freely filtered but not secreted or reabsorbed. Clearance is comparative because only the net handling of a substance can be determined. Inulin is used as an exogenous standard; creatinine an endogenous standard.

Two-thirds of the body’s water is in the ICF; one third in the ECF. The ICF and ECF are in osmotic balance.

The kidneys’ primary functions are (1) to maintain fluid volumes of the body by regulating salt balance and (2) to maintain the osmolarity of the body by regulating water balance.

Reabsorption and secretion of water and solutes is governed by concentration gradients and secondary active transport.

Healthy people use hormones to regulate osmolarity (ADH), to regulate K+ (aldosterone), and to regulate volume (ADH, aldosterone, ANF).

Increased urine excretion above 1mL/min is called diuresis. There are several causes including: water, osmotic and diuretic.

As a result of metabolism, the body has a net production of acids. The kidneys excrete excess H+ combined with urinary buffers such as phosphate (fixed or titratable acid) and ammonia.

The kidneys with the lungs maintain the body’s pH by regulating the HCO3-/CO2 buffer pair. The lungs exert an immediate effect by controlling PCO2; the kidneys exert a slower effect by controlling HCO3- and H+ concentration.

There are four types of acid-base disturbances. They are classified as to the direction of change in pH (acidosis or alkalosis) and by the underlying problem (ventilation or metabolism).

Reabsorption of Solutes

At transport maximum (Tmax) transporters are saturated.

There is alinear uptake of glucose using transporters below 300 mg/100mL plasma.

In DM, glucose remains in the tubule which holds water in the tubule and they cannot concentrate urine.

Aldosterone is released in response to increased K+ and angiotensin II.

Renin => ANG I => ANG II => Aldosterone release.

Volume Loss

Decreased plasma volume stimulates the hypothalamus, via the posterior pituitary, to release ADH (vasopressin), which stimulates the kidneys to release renin. Renin -> ANG I -> ANG II (controlled by ACE). The adrenal is stimulated to release aldosterone which acts on the kidneys to conserve H2O and Na+.

Negatively charged proteins (Hb) trap H+ but do not rid the body of H+.

60-70% of body's total buffering capacity.

Extracellular: Bicarbonate HCO3-

Ventillation keeps normal plasma HCO3- = 24 mEq/L

PCT of Kidney Reabsorbes Filtered HCO3-

PCT reabsorbs 70-80% HCO3-

CD Secretes Filtered H+ or HCO3-

Type A secrete H+ in acidosis (acid urine)

Type B secrete HCO3- in alkalosis (basic urine)

NH4+ Excretion Generates New HCO3-

Glutamine converted to NH4+ which converts to NH3 in CD where it combines with H+ and NH4+ is excreted.

If NH4+ stays in the blood, liver converts it to urea and it shows in blood tests as elevated BUN.

Elevated BUN = kidney problems, collecting ducts aren't working.

How to Analyze Acid-Base Disorders

What is pH of arterial blood?

- normal (pH 7.35-7.45), alkalemia (>pH 7.45, acidemia (<pH 7.35)?

Is it metabolic or respiratory disorder; acidosis or alkalosis?

- examine the HCO3- and PaCO2. Normal values are 24 mEq/L for HCO3- and 40 mm Hg for PaCO2. Compensatory mechanisms can not correct acid-base disorders by themselves, change in pH indicates underlying problem as either acidosis or alkalosis.